Other questions have addressed the question of what exactly a user_defined custom attribute means: the attribute is considered a "system" attribute (note the ambiguous terminology here), and as such it will not appear on the admin as removable because it is considered essential to the operation of Magento.

Specific Context

However, when omitting the system and user_defined configuration values altogether, I found that the attributes I defined defaulted to 'system' => true. When I later attempted to set and save those attributes via a repository, as such: